This invention relates to solenoids and in particular solenoids which are specially adapted for use in mass flow control valves.
Mass flow control valves are used to provide very precise flows of liquids and/or gasses. One common use is in providing the proper flow of gas needed to dope the silicon used for semi-conductors and integrated circuits. Another use of these precise flow control valves is in the manufacture of optical fibers.
The precise flow control needed is provided in a number of ways in the prior art. These include needle valves, motor valves, heater valves, and solenoid valves. In each case there must be constant monitoring of the flow of liquid or gas through the valve and changes made to the valve aperture when the flow has increased or decreased beyond an acceptable level. In selecting a valve for use as a mass flow controller, it is important to keep in mind the environment in which the valve will be operating, how precise the flow must be and the cost of the valve. Additionally, if the gas or liquid flowing through the valve is corrosive, a corrosion resistant valve should be selected.
Solenoid valves presently in common use as mass flow controllers typically include a solenoid having a coil and a ferromagnetic plunger located within the center of the coil. When current flows through the coil, a magnetic field is created which acts to draw the ferromagnetic plunger within the coil. The plunger is typically connected to a valve member such that as the plunger is drawn into the solenoid coil, the valve member is drawn away from the valve aperture in the valve seat, opening the valve further. In order to prevent opening of the valve when no current flows through the coil, the plunger is typically spring-biased to a closed valve position. The more current flowing through the coil the higher the magnetic field and the stronger the force pulling the ferromagnetic plunger into the coil. Thus, it is possible to control the flow through the valve by increasing or decreasing the current flowing through the coil and thus increasing or decreasing the force exerted on the plunger against the biasing spring. The length of the stroke through which the plunger moves is typically of the order of 0.060 inches.
The efficiency of the simple solenoid structure described above can be vastly improved by making some fairly simple modifications. The solenoid, as modified, will be referred to as "the conventional solenoid". It is well known that the placing of a piece of ferromagnetic material in a magnetic field acts to increase the strength of the magnetic field. Typically a stationary ferromagnetic core is placed in the upper portion of the center of the coil. As a result, the magnetic field which acts on the plunger with the same amount of current flowing through the coil is substantially increased over that of the simple solenoid described above without the core. The increased field means an increased force upward on the plunger. This force, for the same amount of current flowing through the coil, can be further increased by surrounding the coil with a ferromagnetic casing. The casing should be very close to and preferably in contact with the upper portion of the core and at the other end of the coil the casing should come as close as possible to the plunger itself. The casing provides a return path for the magnetic flux flowing from the top of the core around the outside of the coil and into the plunger. The ferromagnetic plunger, like the core, acts to increase the magnetic field. This increase is enhanced by the proximity of the ferromagnetic casing.
The efficiency improvements over the simple solenoid made by the above changes are substantial and have made the conventional solenoid a useful means of controlling the flow through a valve. However, for some applications the power requirements, even after incorporating the above efficiency measures into the simple solenoid, are such that the size of the valve structure (including the solenoid coil) is unacceptably large. It has also been found that when conventional solenoid valves are used in vibration-filled environments the controlability of the valve is substantially impaired due to the large mass of the plunger and generally low mechanical frequency response of these valves. If the resonant frequency of the solenoid is generated, for example, due to the nearby use of a vacuum pump, the solenoid casing will begin to move at a different rate than the plunger causing the valve seat to vibrate and in turn changing the valve aperture. Depending on the application and the degree of variation in the flow through the valve this problem can make use of the solenoid valve unacceptable. Additionally, for some applications, the power requirements of the conventional solenoid have been found to be too high.